VSEPR Part 2
Objective:

To use a molecular geometry database to analyze patterns in the shape of molecules when lone pairs are present on the central atom.
Procedure:
1.
Go to website: http://www.stolaf.edu/depts/chemistry/mo/struc/explore.htm
This is a database of actual, experimentally- determined structures. They are displayed TO SCALE (100 pm =
1 cm on the screen). You can rotate them, zoom in, and measure bond lengths and angles.
2.
Click the "Expanded Octet" button at the top, and then the "Pick All" button on the right, and the "Display
List of ___ structures" link to see a list of molecules that fit the criteria.
3.
Click on the blue “View” links to see each molecular shape in the table you have been provided.
4.
Rotate the molecule (click and drag or SHIFT-click and drag to rotate around 1 axis only) so that it is in approximately the same orientation as the Lewis structure in your table.
5.
Measure all bond angles and mark them on your Lewis structure. (Double-click on atom A, then single click on atom B, then double click on atom C to get the bond angle

ABC.)
Read this!
An electron domain is any region around a central atom that contains electrons. All of the following are counted as one electron domain. a) Two electrons in a nonbonding domain (also known as a lone pair) b) Two electrons in a single bond domain c) Four electrons in a double bond domain d) Six electrons in a triple bond domain
For example, the molecule below has 4 total electron domains (1 lone pair + 2 single bonds + 1 double bond).
6.
Cut out each row of your table from Part 1 using a pair of scissors. Add these strips to your molecules from
Part 1 (use only 5 of the molecules from Part 1- one for each geometry).

VSEPR Part 2
7.
Group the molecules according to the total number of electron domains around the central atom.
8.
Use your data to fill in the following table.
Total # electron domains on central atom
2
# of bonding domains
2
3
3
2
# of nonbonding domains
0
0
Molecular Geometry
Smallest bond angle
1
0 4
3 4 1
2 2
5 0
1
5
4
3 2
3 2
6 0
1 6 5
4 3
Analysis:
1.
Ammonia and boron trifluoride both have three atoms bonded to the central atom. However, one adopts a trigonal pyramidal shape, while the other a trigonal planar shape. Suggest an explanation for this difference using VSEPR theory.
2.
Water and ozone both have bent molecular geometries. However, they have very different bond angles.
Suggest an explanation for this difference using VSEPR theory.

VSEPR Part 2
3.
Some chemists use AB x
E y
notation to simplify electron domain information. A stands for the central atom, B stands for the # of bonding domains, and E stands for the # of nonbonding domains (lone pairs). The variables x and y signify the number of bonding and nonbonding domains, respectively.
For example, ammonia has 3 bonding domains and 1 nonbonding domain. Its notation is therefore AB
3
E
1
.
The information is useful since all AB
3
E
1
molecules have the same geometry. a) Give the formula of a compound that could be represented by the notation AB
2
E
2
. b) Label each line of the above table with its proper AB x
E y
notation.

Formula
Lewis structure with labeled bond angles.
VSEPR Part 2
# atoms around central atom
# lone pairs on central atom
Geometry name
BrF
3
H
2
O
I
3
-
NH
3
O
3

SbF
4
-
TeF
5
-
XeF
4
VSEPR Part 2